[0001] The present invention relates to a thermal management device for managing the dissipation
of heat in, for example, electronic equipment and a method of making such a device.
In particular, the invention relates to a thermal management device that has electrical
feed through capability and can act as a direct interface to active elements.
[0002] Electronic and electrical devices are the sources of both power and heat. As is well
known, in order to provide reliable operation of such devices, it is necessary to
maintain stable operating conditions and temperatures. Hence, efficient methods for
heat management and dissipation are essential. Typically this is done by providing
thermal management devices that are arranged adjacent and in contact with the electronic
device or circuit board. Heat generated in the circuit is transferred to and dissipated
in the thermal management device. For optimum efficiency, it is desirable that thermal
management structures have the highest possible thermal conductivity, efficient external
connectivity and appropriate mechanical strength.
[0003] To achieve these objectives in thermally demanding applications, some known devices
encapsulate high thermal conductivity materials into composite structures. However,
these devices often achieve only limited performance, with significant conductivity
losses, typically 40%, and increases in mass and bulk. Examples of such structures
are described in EP 0,147,014, EP 0,428,458, US 5,296,310, US 4,791,248 and EP 0,231,823.
The best thermal management systems available at present have conductivities that
typically do not exceed 1,000W/mK.
[0004] Current technologies do not provide thermal management that is sufficient in many
applications whilst at the same time providing efficient electrical interconnection
between layers or sides of circuit boards. A further problem is that the mass and
volume of known thermal management systems are relatively large. This affects the
overall size of electronic systems in which such devices are incorporated. In this
day and age when the general drive of the electronics industry is towards miniaturisation,
this is highly disadvantageous.
[0005] Thermal management systems are often used as substrates for supports for hybrid electronic
circuits. In one known arrangement, beryllia is used as a heat sink. This has a thermal
conductivity of around 280W/mK at room temperature. On top of this is a layer of dielectric
on which gold contacts are subsequently formed, thereby to enable connection to other
electrical circuits. A disadvantage of this arrangement is that beryllia is a hazardous
material, in fact it is carcinogenic, and is generally difficult to process. In addition,
the dielectric tends to be thick thereby making the overall structure bulky. Furthermore,
partly because of the use of gold as a contact material, the overall structure is
expensive to manufacture,
[0006] An object of the present invention is to provide a thermal management system that
has a high thermal conductivity but a low mass and volume.
[0007] According to a first aspect of the present invention there is provided a thermal
management device comprising anisotropic carbon encapsulated in an encapsulating material
that is applied directly to the carbon and is able to improve the rigidity of the
carbon, preferably wherein the encapsulating material is polyimide or epoxy resin
or acrylic or polyurethane or polyester or any other suitable polymer.
[0008] Preferably, the anisotropic carbon has mosaic or full ordering.
[0009] Preferably, the anisotropic carbon is thermalised pyrolytic graphite that has mosaic
or full ordering. The thermalised pyrolytic graphite may have an in plane thermal
conductivity of 1550-1850W/mK at around room temperature. Typically, the thermalised
pyrolitic graphite has a low value of tensile strength in the orthogonal direction.
[0010] The anisotropic carbon may alternatively be pyrolytic graphite. The pyrolytic graphite
may be in an "as deposited" or partially ordered form. The conductivity of the pyrolytic
graphite may be in the range of 300-420W/mK in one plane. The tensile strength of
the plate may be 1.5K.si in the orthogonal plane.
[0011] Preferably, the anisotropic carbon is a plate. Preferably the carbon plate has a
thickness in the range 100-500µm. The carbon plate may have a thickness in the range
of 200-250µm or 250-300µm or 300-350µm or 350-400µm or 400-450µm or 450-500µm.
[0012] Preferably the material encapsulating the carbon has a low thermal expansion coefficient
and high degradation temperature, such as a polyimide, for example PI 2734 provided
by DuPont (trade mark), where the thermal expansion coefficient is around 13ppm/C
and the degradation temperature is around 500C.
[0013] The coating layer may have a thickness in the range from a few microns to many tens
of microns. Multiple layers of coating may be formed on the carbon in order to build
up a desired thickness.
[0014] A matrix of fine holes, preferably 200µm diameter. may be formed through the carbon
plate, prior to encapsulation. These holes are filled during encapsulation of the
plate. An advantage of this is that it reduces the possibility of internal delamination.
[0015] According to a second aspect of the present invention, there is provided an electrical
system comprising a thermal management device in which the first aspect of the invention
is embodied, on a surface of which electrical contacts and/or devices are provided.
[0016] The devices may be deposited directly on the surface or may be glued using, for example,
a thin layer of liquid glue. Preferably, the devices are encapsulated in polyimide
or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
[0017] Preferably, a plurality of layers of electrical components are provided, each spaced
apart by layers of polyimide. Typically, the electrical contacts are made of thin
film metal, for example aluminium.
[0018] According to a third aspect of the present invention there is provided a method of
fabricating a thermal management device comprising:
applying a coat of encapsulating material, preferably polyimide or epoxy resin or
acrylic or polyurethane or polyester or any other suitable polymer directly to a clean
carbon surface, the encapsulating material being such as to improve the rigidity of
the carbon; and
repeating the foregoing steps until the carbon is encapsulated.
[0019] The method may additionally involve curing the encapsulating material.
[0020] Preferably, the step of applying involves brushing, rolling, dipping, spraying, spinning,
stamping or screen-printing. Preferably, for polyimide, which consists of a single-component,
the step of applying the coating involves brushing the polyimide or applying it using
a roller. For solid phase application a cast can be used. This requires a pre-polymerised
foil of the encapsulating material to be applied directly on to the clean surface.
This can be useful when simple thermal management devices are required with no internal
holes. Preferably, the carbon and cast are compressed within a vacuum and at high
temperature.
[0021] Preferably, the step of applying involves applying multiple layers of encapsulating
material, such as polyimide or epoxy resin or acrylic or. polyurethane or polyester
or any other suitable polymer, until a desired thickness is reached.
[0022] Preferably, the method includes cleaning a surface of the carbon thereby to produce
said clean carbon surface.
[0023] Preferably, the step of cleaning involves using pumice powder under water to remove
loose-materials, followed by drying. Preferably, the step of drying involves drying
the carbon by baking the carbon surface to remove moisture, for example, at 100C for
one hour.
[0024] Preferably the step of cleaning includes degreasing the carbon by, for example, rinsing
it with acetone.
[0025] When polyimide is used, it is preferable that the step of curing involves heating
the carbon to 150C for, for example, 1 hour and subsequently temperature cycling the
board to 150C for 30 minutes, 250C for 30 minutes and finally 300C for 30 minutes.
[0026] In the case of epoxy, this can consist of a single component or else be a double
component mixture. For the single component type then a two stage gluing can be carried
out by firstly drying the glue to remove the solvent at a given temperature, (typically
around 120C) and form a solid phase, and then heating it at a higher temperature typically
around 180° to complete the polymerisation. In the case of double component epoxy,
the initial mixing of the components causes the polymerisation process to begin, and
the process may then need anything between minutes and several hours for the process
to be completed, depending upon the particular epoxy.
[0027] Preferably the method further comprises drilling the carbon with at least one hole
prior to application of the encapsulating material. The at least one hole may be completely
infilled with encapsulating material. The holes may be infilled with encapsulating
material that is mixed with glass fibre spheres, each sphere typically having a diameter
of 30µm. This process may be carried out before the pure polyimide coating is used
to encapsulate the surface of the plate, and can improve the uniformity of coating
thickness across the surface of the plate by preventing the possibility of thinning
occurring around the edges of the initial holes in the plates. In either case, once
the encapsulation process is completed, the said at least one hole is re-drilled,
thereby to provide a through hole that is electrically insulated from the carbon core.
[0028] Preferably, a layer of a conducting material is applied to the at least one hole
to produce electrical connections, thereby to enable electrical connections through
the carbon. Preferably, the conducting material is a metal, for example thin film
aluminium. Alternatively, the edges that define the at least one hole may be coated
with the encapsulating material in such a way as to maintain a passage through the
carbon, thereby to avoid having to conduct the step of drilling through encapsulating
material.
[0029] The method may further involve forming a matrix of fine holes through the carbon.
These holes are of course infilled when the plate is fully encapsulated.
[0030] According to a fourth aspect of the present invention, there is provided a method
of fabricating an electrical component comprising the method of the third aspect of
the present invention and additionally the steps of forming electrical contacts on
at least one surface of the carbon and/or depositing electrical devices thereon.
[0031] The step of depositing may involve fabricating the devices directly on the surface
or forming the devices or a thin film multi-layer circuit containing the devices separately
from the carbon surface and fixing them to that surface. Preferably, the step of fixing
involves applying glue to the devices or the circuit or the carbon surface and pressing
the devices or circuit and the surface together at room temperature and at low vacuum.
[0032] Preferably, the electrical contacts are applied using thin film processing techniques.
using, for example, aluminium.
[0033] Various devices and methods in which the present invention is embodied will now be
described by way of example only and with reference to the following drawings, of
which:
Figure 1 is a cross-section through a carbon plate;
Figure 2 is a cross-section through a plate that has been partially coated with an
encapsulating material, such as polyimide or epoxy resin or acrylic or polyurethane
or polyester or any other suitable polymer;
Figure 3 is a cross-section through a plate that has been fully encapsulated with
the encapsulating material;
Figure 4 is a cross-section through a carbon plate that has been drilled with holes;
Figure 5 is similar cross-section to that of Figure 4 except that the plate has been
coated with encapsulating material;
Figure 6 is a cross-section similar to that of Figure 5 except that holes are formed
through the encapsulating material;
Figure 7 is a cross-section similar to that of Figure 6 in which the plate has been
covered with metal;
Figure 8 is a cross-section similar to that of Figure 7 in which interconnection structures
have been etched on both sides of the plate;
Figure 9 is a cross section through a plate similar to that shown in Figure 3 onto
which a multi-layer electrical circuit has been directly fabricated;
Figure 10 is a cross section similar to that of Figure 9, but in this case the multi-layer
electrical circuit has been fixed to a surface of the carbon plate using epoxy resin;
Figure 11 is similar to Figure 10, except that the multi-layer circuit is mounted
using epoxy on a surface of a carbon plate that has been coated with polyimide;
Figure 12 is a cross section through a structure that is similar to that of Figure
9, except a compensatory layer is included on the back-side of the structure;
Figure 13 is a cross section through a structure that is similar to that of Figure
11, except a compensatory layer is included on the back-side, and
Figure 14 is a top view of a large carbon plate, on which is a plurality processed
sites.
[0034] Figure 1 shows a carbon plate 10. This is typically thermalised pyrolytic graphite
with mosaic or full ordering, with an in plane thermal conductivity (indicated by
arrow A) of 1550-1850W/mK and a thermal conductivity of 8-25W/mK in the orthogonal
direction (indicated by arrow B), with both directions having low values of tensile
strength. This material is friable, breaks easily and so is generally difficult to
handle. In addition, due to its inherent softness and layered nature any contact with
this material results in small traces of it being transferred to the surface it touched.
This is disadvantageous in electrical circuits where any stray shards or pieces of
conducting material can result in electrical shorts being formed.
[0035] The plate 10 may alternatively be pyrolytic graphite in an "as deposited" or partially
ordered form. This material is anisotropic and typically has a thermal conductivity
in the region of 300-420W/mK in one plane (indicated generally by arrow A in Figure
1) and 3W/mK in the orthogonal direction (indicated by arrow B in Figure 1) with respective
tensile strengths of 14Ksi and 1.5Ksi.
[0036] The plate 10 may have a thickness in the range 100-500µm, preferably 200µm, although
could have any thickness suitable for a given application.
[0037] In order to form a thermal management board that has a high thermal conductivity
and is sufficiently mechanically rigid to enable electrical components to be mounted
thereon, the plate 10 is directly coated with an encapsulating material. Suitable
encapsulating materials include polyimide or epoxy resin or acrylic or polyurethane
or polyester 12 or any other such polymer that can be applied directly to the carbon
surface and is able to improve the rigidity of the plate without reducing significantly
its thermal conductivity. One example of a suitable polyimide is PI 2734, provided
by DuPont (trade mark).
[0038] Prior to coating, a matrix of fine holes may be formed through the plate (not shown).
The diameter of the holes is typically 200µm. This has the advantage of reducing the
possibility of internal delamination.
[0039] In order to carry out the encapsulating process, the surface of the plate 10, is
firstly brushed under water with pumice powder, thereby to remove any loose material.
The plate 10 is dried for one hour at 100°C and degreased with, for example, acetone.
A coat of one of the encapsulating materials, for example, PI 2734 approximately 8µm
thick is then applied to one surface of the plate using a brush and the plate 10 is
heated for about one hour at 150°C to partially polymerize the polyimide. This results
in one side of the plate 10 being coated with the polyimide 12, as shown in Figure
2.
[0040] The foregoing steps are then repeated on each side of the plate 10 until it is fully
encapsulated and the desired thickness of the polyimide is reached, as shown in Figure
3, thereby forming a thermal management board 13. Generally, these steps are carried
out on alternate surfaces so that the flatness of the board can be preserved. It is
important at this stage to ensure that all sides and edges of the plate are covered.
If, however, it is necessary to contact the graphite for some reason, small holes
may be left in the polyimide, although these would be in-filled when the appropriate
contact is made. Finally, the board 13 is thermally cycled so that it is cured. The
thermal cycling for a carbon plate encapsulated by PI 2734 typically involves heating
the board 13 to 150°C for 30 minutes, 200°C for 30 minutes, 250°C for 30 minutes and
300°C for 30 minutes. If a high level of flatness is required, then during the curing
stage, the board surfaces are compressed within a press at low vacuum.
[0041] The encapsulation process is adapted to suit the specification and geometrical form
of the required thermal management structure. For example, if the geometric form of
the substrate includes internal holes and/or a complex perimeter with the need for
all surfaces and edges to be coated uniformly it is preferred to apply the encapsulating
material to the cleaned carbon surfaces using a brush or a roller. This allows all
surfaces and edges to be coated as required. Alternatively, the substrate could be
coated using techniques such as dipping, spinning, spraying, stamping or screen printing.
The drying, heating and optional low vacuum pressing steps are then carried out in
the same manner and sequence as previously described.
[0042] The processing steps for all the encapsulating materials are essentially the same,
but as will be appreciated the temperatures used to cause partial polymerization and
curing vary. For example in the case of epoxy resin, if type G10 FR4 is used, once
the carbon is completely encapsulated it is heated typically to 180C for about one
hour to cure the resin and thereby form the thermal management board. If required,
further epoxy layers can be added by repeating the steps of applying the resin and
heating the board to form an encapsulation layer of the required thickness.
[0043] According to another encapsulation technique that uses epoxy resin, for example STYCAST
(type 1266) which is a two component epoxy resin, all the resin processing steps can
be carried out at room temperature. This minimises the possibilities of generating
internal stresses or internal delamination of the substrate. The preparations of the
substrate surfaces prior to encapsulation are carried out as previously described.
The technique for applying the epoxy resin to the surface of the substrate, using
for example screen printing or a brush or roller, is again determined by the same
considerations of substrate geometry and form.
[0044] In the case of the room temperature processing, where curing times can be between
minutes and several hours, depending upon the properties of the particular epoxy resin,
the encapsulation process typically has a sequence of curing procedures. An initial
low vacuum environment encourages degassing to produce a bubble-free coating. This
is followed by the combined application to the board of both a low vacuum and high
surface pressure. In this way, a high level of mechanical flatness can be provided
for the encapsulated thermal management structure.
[0045] The process of encapsulating the plate 10 in any one of the described encapsulation
materials maintains the thermal conductivity of the plate at substantially its pre-coating
level. For example, when thermalised pyrolytic graphite is used, the resulting thermal
management board has an in-plane thermal conductivity of typically 1700W/mK at room
temperature. It will be appreciated that for lower temperatures the conductivity is
likely to be higher. This is advantageous. Another advantage of the process of encapsulating
the carbon is that the flatness of the thermal management board can be maintained
at typically plus or minus 5µm across a plate that is 100mm by 100mm, provided the
original material is suitably flat.
[0046] Using the encapsulation process described above it is possible to encapsulate, for
example, a graphite plate 10 having a thickness of 200µm in a polyimide or epoxy resin
or acrylic or polyurethane or polyester layer having a thickness in the range of 8-30µm,
preferably 15µm. This results in a thermal management board 13 having a total thickness
in the range of 208-230µm. Encapsulating the plate in this amount of material results
in a board having a tensile strength that is significantly higher than that of the
original carbon plate, thereby making the board sufficiently strong for it to be handled
easily. This is done with a negligible increase in volume and loss of thermal conductivity.
This is unexpected and advantageous.
[0047] In many applications, thermal management devices are sandwiched between layers of
printed circuit boards. Hence, it is advantageous to be able to allow direct electrical
interconnection between opposing sides of the device. In order to achieve this in
the present case, prior to encapsulation, a matrix of holes is formed in the graphite
plate 10 by, for example, drilling. This is shown in Figure 4. The holes 14 should
each have a diameter that is greater than the desired final diameter. Typically, the
diameter of the holes 14 formed at this stage would be at least 200µm greater than
the desired diameter. The holes 14 can of course be formed in any desired layout.
Polyimide or epoxy resin or acrylic or polyurethane or polyester 12 or any other suitable
polymer is then applied to the plate 10 in order to coat its surfaces and fill in
the holes 14, as shown in Figure 5. If desired, the holes could in fact be infilled
with a mixture of the encapsulating material, for example polyimide, and glass spheres.
This process can be carried out before the pure polyimide is used to encapsulate the
surface of the plate. This improves the uniformity of the coating thickness across
the surface of the plate, by preventing the possibility of thinning occurring around
the edges of the initial holes. Once the plate is fully encapsulated, it is then processed
as described above thereby to provide a rigid and highly thermally conducting board.
[0048] In order to provide electrical connections through the board, the in-filled holes
14 are re-drilled to form holes 16 of a smaller diameter, typically 100µm or greater,
as shown in Figure 6. In this way passages are formed through the board but the graphite
10 material remains encapsulated in the polyimide or resin or acrylic or polyurethane
or polyester 12 or other suitable polymer and so electrically insulated. Metal 18,
such as aluminium is then deposited on both sides of the board, as shown in Figure
7, typically using thin film aluminium processing techniques. Interconnection structures
20 are subsequently etched using standard techniques on both sides of the board, as
shown in Figure 8. In this way, a board 22 is produced having metalised holes through
an encapsulated carbon plate, the metal of the holes being entirely insulated from
the carbon 10.
[0049] The encapsulated thermal management board 13,22 can be used as an interface to various
assemblies. For example, it can be used for direct thermal management of ceramic substrates
such as alumina, beryllia and aluminium nitride, or metal substrates such as beryllium.
This is achieved by applying, for example, a thin layer of liquid epoxy resin to one
surface of the ceramic substrate, heating the substrate to 125°C to polymerize the
resin, and then positioning the substrate on the carbon plate 10 or thermal management
board 10,22. A high pressure, low vacuum pressing at 180°C is then applied to produce
a bubble-free interface with a thickness of only a few microns. An alternative process
is to coat the ceramic or metal substrate with a thin layer of liquid epoxy glue (typically
a few microns thick), position it onto the anisotropic carbon plate or the thermal
management board and attach it by allowing the epoxy to polymerise under pressing
and low vacuum at room temperature, in order to produce a bubble-free interface.
[0050] The thermal management board 13,22 can also be used as a substrate for the custom
design of thin film multi-layer circuits using alternating layers of vacuum deposited
aluminium and polyimide. The aluminium 24 may be directly deposited onto the polyimide
or resin or acrylic or polyurethane or polyester of the board 13,22 typically using
thin film aluminium techniques so that layers having thicknesses of 5µm can be deposited.
Figure 9 shows aluminium 24 deposited onto a layer of epoxy resin 26 which is in turn
deposited on one surface of plate 10. Because the coated surface of the plate 10 is
flat, the resolution of the lithography used to deposit the aluminium 24 is good.
This means that small features can be readily defined. Polyimide 28 is then applied
over the aluminium by spinning or screen-printing. Hence, the thickness of the polyimide
layer 28 can be, for example, as little as 8µm. Using standard fabrication techniques,
holes are then defined through the polyimide 28 in appropriate places so that subsequent
layers of metal 30 that fill these holes can provide electrical contact to the aluminium
24. Between the subsequent layers of metal 30 are, typically, layers of polyimide
28. Of course this processing could be done on opposing sides of the plate 10 thereby
to provide a double sided electrical component with an intrinsic thermal management
capability.
[0051] The thin-film multi-layer circuits can also be fabricated on alternative substrates,
for example aluminium, and subsequently separated chemically. These circuits or other
custom-designed multi-layer circuits which may be fabricated on polyimide layers or
epoxy resin based layers 31 can also be interfaced to the initial anisotropic carbon
plate by applying, for example, a thin layer of liquid epoxy glue 32 (typically a
few microns thick) to the plate 10, placing the multi-layer circuit on that surface
and allowing the epoxy to polymerise under pressing and low vacuum at room temperature,
in order to produce a bubble-free interface. A device that has been fabricated by
applying epoxy resin 32 to a carbon plate 10 is shown in Figure 10. In this case the
epoxy resin 32 acts both as a fixing agent to secure the multi-layer circuit to the
carbon plate 10 and additionally as the material for encapsulating the carbon plate.
In contrast, Figure 11 shows a multi-layer circuit that is interfaced using epoxy
resin 32 with a surface of a carbon plate 10 that has been coated with, for example,
polyimide 36.
[0052] In hybrid structures fabricated using any of the processes described above, changes
in the temperature can cause variations in the lengths of the structural component
layers. The changes in length for the encapsulated board are different from those
of the materials that form the attached multi-layer hybrid structure. This affect
degrades the overall surface flatness, which can be a disadvantage in some applications.
It has been found, however, that optimum flatness can be maintained, over a range
of temperatures, typically 100C, by depositing a compensatory layer of encapsulating
material on the opposite side of the thermal management board to that which carries
the hybrid. This compensatory layer should be of the same material and substantially
the same thickness as the layers of material that form the multi-layer hybrid structure.
In this way, each side of the board has approximately the same co-efficient of thermal
expansion and the overall flatness of the board does not vary substantially.
[0053] The compensatory layer can be mounted either by building up additional layers of
encapsulating material on the board until the desired thickness is achieved or alternatively
by gluing a cast of the material onto the surface of the board in a similar manner
as previously described. As an example, Figure 12 shows the structure of Figure 9
onto which has been deposited a compensatory layer of polyimide 37 that has a thickness
that is roughly the same as the combined thickness of layers 28 of Figure 10, assuming
in this case that the thickness of the hybrid structure is dominated by the layers
of polyimide 28. As a further example, Figure 13 shows the structure of Figure 11,
onto which has been glued a compensatory layer of polyimide 37, which has a thickness
that is roughly the same as the combined thickness of the layers 28 and 31 of the
structure of Figure 11. Again this assumes that the thickness of the hybrid structure
is dominated by the layers 28 and 31.
[0054] To provide additional rigidity to the composite structure and/or to protect the edges
against impact or delamination, the anisotropic carbon plate can be inserted within
a surrounding thin frame, which is preferably made of material having the same co-efficient
of thermal expansion as the structure, for example, carbon fibre. In this way, a single
flat surface is provided that can be coated and attached to the multi-layer circuit
in the room temperature process described above.
[0055] Hybrid devices that include, for example, multi-layer circuits and a thermal management
board can be made in various ways. In one technique, a plurality of such devices is
fabricated from one large carbon plate. Figure 14 shows such a plate 38 on which are
six processing sites 40. Each processing site is coated with, for example, polyimide,
onto which the multi-layer circuits can be either directly deposited or fixed using
epoxy glue. Once the processing at each site is completed, the plate 38 is cut up
to form six discrete devices. The uncoated sides of the carbon plate are then processed
as previously described to ensure complete encapsulation of the carbon and formation
of a thermal management board. An advantage of this particular technique is that problems
associated with the edges of the carbon plate 38 are avoided.
[0056] These procedures allow the thermal conductivity and low mass property of the initial
thermal management structure to be preserved after interfacing with the custom-made
multi-layer circuits.
[0057] As mentioned previously in some known applications where relatively high thermal
conductivity is required, beryllia substrates with a layer of dielectric formed thereon
have been used, and gold contacts are deposited onto the dielectric. However, the
hybrid electronic device that uses the thermal management device in which the present
invention is embodied provides significantly higher thermal conductivity with a significant
reduction in cost. Furthermore, because the materials involved are not hazardous,
the fabrication of such devices is less problematic.
[0058] The process as described above allows the fabrication of electronic assemblies with
high component densities constructed on a high thermal conductivity, low mass, graphite
plate or core having the possibility of customised electrical interconnections between
its opposing faces. This is achieved without the use of hazardous materials.
[0059] The thermal management board that uses thermalised pyrolytic graphite has an optimal
in-plane thermal conductivity typically of 1550-1850mK at room temperature, whilst
at the same time having a low mass and easy to handle structure. In addition, the
substrates can be readily used as interfaces between other circuits. Furthermore,
any geometry of the carbon can be used prior to encapsulation so that the thermal
management device can be custom made for each particular application. This is advantageous
because it means that the application of the process is not significantly limited.
[0060] In the use of thermal management structures for cooling electrical systems, thermal
grease is often used as an interface. It is envisaged that a thermal management device
in which the invention is embodied could be used in place of the grease, it being
appreciated that the device used in such an application should be relatively thin.
[0061] The skilled person will appreciate that variations of the disclosed arrangements
are possible without departing from the invention. Accordingly, the above description
of several embodiments is made by way of example and not for the purposes of limitation.
In addition, it will be clear to the skilled person that minor modifications can be
made without significant changes to the concepts described above.
1. A thermal management device comprising anisotropic carbon encapsulated in an encapsulating
material that is applied directly to the anisotropic carbon and improves the rigidity
of the carbon, preferably wherein the encapsulating material is polymide or epoxy
resin or acrylic or polyurethane or polyester or any other suitable polymer.
2. A thermal management device as claimed in claim 1, wherein the anisotropic carbon
has mosaic or full ordering.
3. A thermal management device as claimed in claim 1 or 2, wherein the anisotropic carbon
is thermalised pyrolytic graphite, and preferably wherein the thermalised pyrolytic
graphite has an in plane thermal conductivity in the range of 1550-1850W/mK at room
temperature, and/or wherein the thermalised pyrolytic graphite has a low value of
tensile strength in the orthogonal direction.
4. A thermal management device as claimed in claim 1, wherein the anisotropic carbon
is pyrolytic graphite, and preferably wherein the pyrolytic graphite is in an "as
deposited" or partially ordered form, and/or wherein the conductivity of the pyrolytic
graphite is in the range of 300-420W/mK in one plane, and/or wherein the tensile strength
of the carbon is 1.5Ksi in the orthogonal plane.
5. A thermal management device as claimed in any one of the preceding claims, wherein
the anisotropic carbon is a plate, and preferably wherein the carbon plate has a thickness
in the range 100-500µm, preferably 200-250µm or 250-300µm or 300-350µm or 350-400µm
or 400-450µm or 450-500µm.
6. A thermal management device as claimed in any one of the preceding claims, wherein
the material encapsulating the carbon has a low thermal expansion coefficient and
high degradation temperature, or wherein the encapsulating layer has a thickness in
the range from a few microns to many tens of microns, or wherein multiple layers of
encapsulating material are deposited on the carbon in order to build up a desired
thickness.
7. A thermal management device as claimed in any one of the preceding claims, wherein
a matrix of fine holes, preferably each hole having a diameter of 200µm diameter,
is formed through the carbon, and preferably wherein the holes are filled during encapsulation
of the plate.
8. An electrical system comprising a thermal management device as claimed in any of the
preceding claims, on a surface of which electrical contacts and/or devices are provided.
9. An electrical system as claimed in claim 8, wherein the devices are deposited directly
on the surface of the thermal management device or are glued using, for example, a
thin layer of liquid glue, and/or wherein the devices are encapsulated in polymide
or epoxy resin or acrylic or polyurethane or polyester or any other suitable polymer.
10. An electrical system as claimed in any one of claims 8 or 9, wherein a plurality of
layers of electrical components are provided, and preferably wherein each layer of
electrical components is spaced apart by layers of encapsulating material, preferably
polymide.
11. An electrical system as claimed in any one of claims 8 to 10, wherein the electrical
contacts are made of thin film metal, preferably aluminimum.
12. A method of fabricating a thermal management device comprising:
applying a coat of encapsulating material, preferably polymide or epoxy resin or acrylic
or polyurethane or polyester or any other suitable polymer directly to a clean carbon
surface,
repeating the foregoing steps until the carbon is encapsulated.
13. A method as claimed in claim 12 that further involves curing the encapsulating material.
14. A method as claimed in claim 12 or claim 13, wherein the step of applying involves
brushing, rolling, dipping, spraying, spinning, stamping or screen-printing, and preferably
where for polyimide, which consists of a single-component, the step of applying the
coating involves brushing the polyimide or applying it using a roller, or wherein
for solid phase application a cast is used, and preferably wherein the carbon and
cast are pressed together within a vacuum and at high temperature.
15. A method as claimed in any one of claims 12 to 14, wherein multiple layers of encapsulating
material are applied until a desired thickness is reached.
16. A method as claimed in any one of claims 12 to 15 including cleaning a surface of
the carbon thereby to produce said clean carbon surface, and preferably wherein the
step of cleaning involves using pumice powder under water to remove loose materials,
followed by drying, and preferably wherein the step of drying involves drying the
carbon by baking the carbon surface to remove moisture, and preferably wherein the
step of drying involves baking the carbon at 100C for one hour.
17. A method as claimed in claim 16, wherein the step of cleaning involves degreasing
the carbon, preferably by rinsing it with acetone.
18. A method as claimed in any one of claims 12 to 17, wherein when polyimide is used,
the step of curing involves heating the carbon to substantially 150C for, preferably,
1 hour and subsequently temperature cycling the board to 150C for 30 minutes, 250C
for 30 minutes and finally 300C for 30 minutes.
19. A method as claimed in any one of claims 12 to 18, further comprising drilling the
carbon with at least one hole prior to application of the encapsulating material.
20. A method as claimed in claim 19, wherein the at least one hole is infilled with encapsulating
material, and preferably wherein the holes are infilled with encapsulating material
that is mixed with glass fibre spheres, each sphere typically having a diameter of
30µm, or wherein the infilled holes are drilled thereby to provide through passages
that are electrically isolated from the carbon, and preferably wherein a layer of
a conducting material is applied to the at least one through passage to produce electrical
connections, thereby to enable electrical connections through the carbon, and preferably
wherein the conducting material is a metal, preferably thin film aluminimum.
21. A method as claimed in claim 19, wherein edges that define the at least one hole are
coated with the encapsulating material in such a way as to maintain a passage through
the carbon.
22. A method as claimed in any one of claims 12 to 21, further involving forming a matrix
of fine holes through the plate.
23. A method of fabricating an electrical component comprising the method of as defined
in any one of claims 12 to 22, additionally comprising the steps of forming electrical
contacts on at least one surface of the carbon and/or depositing electrical devices
thereon, and preferably wherein the step of depositing may involve fabricating the
devices directly on the surface or forming the devices or a thin film multi-layer
circuit containing the devices separately from the carbon surface and fixing them
to that surface, and preferably wherein the step of fixing involves applying epoxy
glue to the devices or the circuit or the carbon surface and pressing the devices
or circuit and the surface together at room temperature and at low vacuum, and preferably
wherein the electrical contacts are applied using thin film processing techniques,
preferably using aluminimum.